The Top 8 Reasons Why Your “Check Engine” Light May Go On

The “Check Engine” light is the little glowing indicator on your dash tells you something is wrong with your car. Automotive professional often refer to the Check Engine Light as the “CEL”. If you drive an older car, chances are pretty good that you’ve seen your CEL light go on. In fact, a recent CarMD survey suggests that roughly 10 percent of all vehicles on the road have their CEL on now. But what causes that light to turn on? Based on an analysis of 160,000 repairs by CarMD, here are the top 8 reasons a CEL can turn on.


1) Faulty oxygen sensor: O2 sensors measures the amount of oxygen in a car’s exhaust to help the car’s computer inject the appropriate amount of fuel into the engine. A failed sensor can throw off a vehicle’s mpg by as much as 40 percent.

2) Faulty catalytic converter: Generally the catalytic converter fails only after something else goes bad and the engine’s exhaust becomes laded with oil or raw gas. Think bad piston rings or a malfunctioning ignition system.

3) Loose gas cap: Gas will evaporate around the opening of a loose gas cap. There are sensors in the gas system that look for vapor leaks such as this and will trigger the check engine light when they are found. If your check engine light comes on, the first thing to do is check to see if your gas cap has been tightened.

4) Bad spark plug(s): If your car isn’t firing on all cylinders, you probably have a bad spark plug or two and this means you’re wasting gas. This symptom often occurs because of bad spark plug wires too.

5) Faulty ignition coil: No coil means no spark, and internal combustion engines need spark to run. The coil can go bad by operating under high temperatures or just by getting old.

6) Bad mass air flow sensor: This is the sensor that meters the engine’s incoming air and determines how much fuel to inject. If it goes bad, your car’s fuel efficiency can drop up to 25 percent. Fortunately mass air flow sensors are relatively easy to replace and not terribly expensive.

7) Non-compatible aftermarket alarm: Certain aftermarket parts work better than others — make sure aftermarket alarms are compatible and are installed by a qualified technician.

8) Leaky vacuum hoses on EVAP system: Loose hoses mean evaporating fuel will not reach its vented destination. This often triggers your check engine light. This is not uncommon on older vehicles because rubber hoses designed to vent gas fumes deteriorate and need replacement.

Source: Deery Bros of Iowa City

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